Solar Gas-Phase CO2 Hydrogenation by Multifunctional UiO-66 Photocatalysts.

Solar-assisted CO2 conversion into fuels and chemical products involves a range of technologies aimed at driving industrial decarbonization methods. In this work, we report on the development of a series of multifunctional metal-organic frameworks (MOFs) based on nitro- or amino-functionalized UiO-66(M) (M: Zr or Zr/Ti) supported RuOx NPs as photocatalysts, having different energy band level diagrams, for CO2 hydrogenation under simulated concentrated sunlight irradiation. RuOx(1 wt %; 2.2 ± 0.9 nm)@UiO-66(Zr/Ti)-NO2 was found to be a reusable photocatalyst, to be selective for CO2 methanation (5.03 mmol g-1 after 22 h;, apparent quantum yield at 350, 400, and 600 nm of 1.67, 0.25, and 0.01%, respectively), and to show about 3-6 times activity compared with previous investigations. The photocatalysts were characterized by advanced spectroscopic techniques like femto- and nanosecond transient absorption, spin electron resonance, and photoluminescence spectroscopies together with (photo)electrochemical measurements. The photocatalytic CO2 methanation mechanism was assessed by operando FTIR spectroscopy. The results indicate that the most active photocatalyst operates under a dual photochemical and photothermal mechanism. This investigation shows the potential of multifunctional MOFs as photocatalysts for solar-driven CO2 recycling.

Impact of the Si/Al ratio on the ethanol/water coadsorption on MFI zeolites revealed using original quantitative IR approaches.

Advanced IR vibrational spectroscopic techniques, e.g., using a coupled gravimetric-IR surface analyzer (AGIR) and a high-throughput in situ IR cell (Carroucell), have been used for the quantitative studies of the adsorption and coadsorption of ethanol and water on MFI zeolites with different Si/Al ratios. The AGIR coupling is a powerful tool for the accurate determination of the molar adsorption coefficients during coadsorption experiments since their evaluation is based on the measurement of the exact amount of adsorbed species. The use of the Carroucell set up allows characterizing all the samples simultaneously, strictly in the same gaseous and temperature environment. The molar absorption coefficients of pure adsorbed ethanol and water are determined: their values are constant whatever the Si/Al ratio of the MFI zeolites. Moreover, these coefficients are found to be identical in the case of the water-ethanol coadsorption experiments. Their use allows obtaining the exact quantity of each adsorbate specie in the binary system. At low partial pressures, the unary water adsorption experiments suggest that the amount of adsorbed water results mainly from the preferential adsorption on Brønsted acid sites in tetrameric clusters. In contrast, the adsorption of EtOH occurs on both silanol groups and Brønsted acid sites (BASs). The effect of the Si/Al ratio is only observed at relatively low partial pressures. The effect of the Si/Al ratio on the ethanol adsorption capacity is also investigated. This study directs the choice of an appropriate zeolite once it is used in membranes for drying ethanol.

Selective Photocatalytic Dehydrogenation of Formic Acid by an In Situ-Restructured Copper-Postmetalated Metal-Organic Framework under Visible Light.

Formic acid is considered as one of the most promising liquid organic hydrogen carriers. Its catalytic dehydrogenation process generally suffers from low activity, low reaction selectivity, low stability of the catalysts, and/or the use of noble-metal-based catalysts. Herein we report a highly selective, efficient, and noble-metal-free photocatalyst for the dehydrogenation of formic acid. This catalyst, UiO-66(COOH)2-Cu, is built by postmetalation of a carboxylic-functionalized Zr-MOF with copper. The visible-light-driven photocatalytic dehydrogenation process through the release of hydrogen and carbon dioxide has been monitored in real-time via operando Fourier transform infrared spectroscopy, which revealed almost 100% selectivity with high stability (over 3 days) and a conversion yield exceeding 60% (around 5 mmol·gcat-1·h-1) under ambient conditions. These performance indicators make UiO-66(COOH)2-Cu among the top photocatalysts for formic acid dehydrogenation. Interestingly, the as-prepared UiO-66(COOH)2-Cu hetero-nanostructure was found to be moderately active under solar irradiation during an induction phase, whereupon it undergoes an in-situ restructuring process through intraframework cross-linking with the formation of the anhydride analogue structure UiO-66(COO)2-Cu and nanoclustering of highly active and stable copper sites, as evidenced by the operando studies coupled with steady-state isotopic transient kinetic experiments, transmission electron microscopy and X-ray photoelectron spectroscopy analyses, and Density Functional Theory calculations. Beyond revealing outstanding catalytic performance for UiO-66(COO)2-Cu, this work delivers an in-depth understanding of the photocatalytic reaction mechanism, which involves evolutive behavior of the postmetalated copper as well as the MOF framework over the reaction. These key findings pave the way toward the engineering of new and efficient catalysts for photocatalytic dehydrogenation of formic acid.

In situ and Operando Spectroscopies in Photocatalysis: Powerful Techniques for a Better Understanding of the Performance and the Reaction Mechanism.

In photocatalysis, a set of elemental steps are involved together at different timescales to govern the overall efficiency of the process. These steps are divided as follow: (1) photon absorption and excitation (in femtoseconds), (2) charge separation (femto- to picoseconds), (3) charge carrier diffusion/transport (nano- to microseconds), and (4 and 5) reactant activation/conversion and mass transfer (micro- to milliseconds). The identification and quantification of these steps, using the appropriate tool/technique, can provide the guidelines to emphasize the most influential key parameter that improve the overall efficiency and to develop the "photocatalyst by design" concept. In this review, the identification/quantification of reactant activation/conversion and mass transfer (steps 4 and 5) is discussed in details using the in situ/operando techniques, especially the infrared (IR), Raman, and X-ray absorption spectroscopy (XAS). The use of these techniques in photocatalysis was highlighted by the most recent and conclusive case studies which allow a better characterization of the active site and reveal the reaction pathways in order to establish a structure-performance relationship. In each case study, the reaction conditions and the reactor design for photocatalysis (pressure, temperature, concentration, etc.) were thoroughly discussed. In the last part, some examples in the use of time-resolved techniques (time-resolved FTIR, photoluminescence, and transient absorption) are also presented as an author's guideline to study the elemental steps in photocatalysis at shorter timescale (ps, ns, and µs).

Unraveling the Origin of Photocatalytic Deactivation in CeO2/Nb2O5 Heterostructure Systems during Methanol Oxidation: Insight into the Role of Cerium Species.

The study provides deep insight into the origin of photocatalytic deactivation of Nb2O5 after modification with ceria. Of particular interest was to fully understand the role of ceria species in diminishing the photocatalytic performance of CeO2/Nb2O5 heterostructures. For this purpose, ceria was loaded on niobia surfaces by wet impregnation. The as-prepared materials were characterized by powder X-ray diffraction, nitrogen physisorption, UV-visible spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. Photocatalytic activity of parent metal oxides (i.e., Nb2O5 and CeO2) and as-prepared CeO2/Nb2O5 heterostructures with different ceria loadings were tested in methanol photooxidation, a model gas-phase reaction. Deep insight into the photocatalytic process provided by operando-IR techniques combined with results of photoluminescence studies revealed that deactivation of CeO2/Nb2O5 heterostructures resulted from increased recombination of photo-excited electrons and holes. The main factor contributing to more efficient recombination of the charge carriers in the heterostructures was the ultrafine size of the ceria species. The presence of such highly dispersed ceria species on the niobia surface provided a strong interface between these two semiconductors, enabling efficient charge transfer from Nb2O5 to CeO2. However, the ceria species supported on niobia exhibited a high defect site concentration, which acted as highly active recombination centers for the photo-induced charge carriers.

In Situ FTIR Reactor for Monitoring Gas-Phase Products during a (Photo)catalytic Reaction in the Liquid Phase.

Various catalytic and photocatalytic reactions in the liquid phase give rise to gas products. Therefore, the identification and quantification of these products are of high importance and are even essential for some reactions. In this paper, a new in situ FTIR reactor is designed and used for analyzing the gas headspace of a (photo)catalytic reaction in solution. It allows the identification and quantification of the gas-phase products of a liquid reaction under operating conditions and in real time. The new reactor has been tested in three representative photocatalytic reactions widely studied as model reactions in the liquid phase: i.e., (i) decomposition of formic acid, (ii) oxidation of methylene blue, and (iii) reduction of CO2. The validity of the results has been confirmed by analyzing the headspace at the end of the reaction using gas chromatography technique. The new reactor opens the possibility to follow online the (photo)catalyst activity. This is useful for ensuring the stability of the catalyst and studying the evolution of the selectivity during the reaction. The nondestructive behavior of the FTIR technique allows its coupling with other techniques for obtaining complementary results. The new reactor setup is easy to handle and to ship and is very efficient, which makes it very suitable for performing complementary, fast and/or preliminary studies.

Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light.

Sub-nanometer silver clusters that exhibit discrete electronic structure with molecular-like properties are highly desirable in various technologies. However, the methods for their preparation suffer from limitations related with the reproducibility and particles uniformity and/or the possibility of the scale-up. Another critical drawback is that free sub-nanometer silver clusters tend to aggregate into larger particles. In this work, a new approach that successfully overcomes the above limitations is developed. It allows, for the first time, an ultrafast preparation of sub-nanometer silver particles with high abundance, uniformity (7 Å), and stability into the cages of nanosized zeolite crystals. The new method consists of UV excitation of a water suspension of nanozeolite containing photoactive vanadate clusters in the presence of ethanol (as an electron donor) and silver precursor. The characteristic features of sub-nanometer silver particles are presented, and the mechanism of their formation is discussed. Sub-nanometer Ag clusters exhibit exceptional photocatalytic activity and selectivity in the reforming of formic acid to H2 and CO2 under visible light.

High-Visible-Light Photoactivity of Plasma-Promoted Vanadium Clusters on Nanozeolites for Partial Photooxidation of Methanol.

Cold VCl3-plasma is employed for the preparation of highly dispersed vanadium oxide clusters on nanosized zeolite. Different types of zeolites, such as EMT, FAU (z.X), and Beta, are used. The activity of the prepared catalysts is studied in the selective photooxidation of methanol under polychromatic visible and UV irradiations. The physicochemical properties and catalytic performance of plasma-treated zeolite Beta (P-V2O5@Beta) catalyst is compared with zeolite Beta (V2O5@Beta) and amorphous silica (V2O5@SiO2) impregnated vanadium oxide catalysts. Pure V2O5 is used as a reference material. The set of catalytic data shows that plasma-prepared zeolite Beta based catalyst displays the highest activity. Complementary characterization techniques including XRD, N2-sorption, FTIR, ionic exchange, pyridine adsorption, Raman, NMR, TPR, and EDX-TEM are used to study the impact of the preparation approach on the physicochemical properties and catalytic performance of photocatalysts.

On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study.

This work is a mechanistic study of total and partial methanol photooxidation using operando FTIR coupled to gas phase analysis techniques (gas-IR and MS). Methoxy and formate/formyl species play a key role in the reaction. Methoxy species are formed by thermal and photochemical dissociation of methanol. The formation of methylformate is favored by a high surface coverage by methoxy species. Surface and/or bridged oxygen atoms are also important actors. Steady State Isotopic Transient Kinetic Analysis (SSITKA) experiments showed that the limiting step is the conversion of chemisorbed formyl/formate and that methylformate is a secondary product from a reaction between methoxy and neighboring formyl species. Methanol concentration, among other reaction parameters, influences greatly the selectivity of photooxidation.

Photochemical preparation of silver nanoparticles supported on zeolite crystals.

A facile and rapid photochemical method for preparing supported silver nanoparticles (Ag-NPs) in a suspension of faujasite type (FAU) zeolite nanocrystals is described. Silver cations are introduced by ion exchange into the zeolite and subsequently irradiated with a Xe-Hg lamp (200 W) in the presence of a photoactive reducing agent (2-hydroxy-2-methylpropiophenone). UV-vis characterization indicates that irradiation time and intensity (I0) influence significantly the amount of silver cations reduced. The full reduction of silver cations takes place after 60 s of a polychromatic irradiation, and a plasmon band of Ag-NPs appears at 380 nm. Transmission electron microscopy combined with theoretical calculation of the plasmon absorbance band using Mie theory shows that the Ag-NPs, stabilized in the micropores and on the external surface of the FAU zeolite nanocrystals, have an almost spheroidal shape with diameters of 0.75 and 1.12 nm, respectively. Ag-NPs, with a homogeneous distribution of size and morphology, embedded in a suspension of FAU zeolites are very stable (∼8 months), even without stabilizers or capping agents.

Incorporation of clusters of titanium oxide in Beta zeolite structure by a new cold TiCl4-plasma process: physicochemical properties and photocatalytic activity.

A new post-synthetic approach, involving cold plasma treatment, was employed for the preparation of TiO2-Beta zeolite. Zeolite Beta nanoparticles were first subjected to plasma induced deposition of TiCly (with y≤ 3), which were further converted into TiOx (with x≤ 2) upon O2-plasma treatment. Different steps of the new elaborated plasma approach were monitored using in situ FTIR spectroscopy. D2O isotopic exchange was used in order to shed light on the formation of Si-O-Ti bonds induced by TiCl4-plasma followed by O2-plasma treatments. The obtained TiO2-Beta materials were studied by a set of complementary characterization techniques including FTIR, TEM, SEM-EDS, XRD, N2 sorption, NMR and UV-Vis. The silanol content and the acidic properties of TiO2-Beta composites were also studied. The elaborated materials were tested as photocatalysts for methanol photooxidation in the gas phase. TiO2-Beta presents a methanol photooxidation rate 8 times higher than a conventional P25-TiO2 catalyst under UV irradiation.

Quantification of silanol sites for the most common mesoporous ordered silicas and organosilicas: total versus accessible silanols.

IR and NMR spectroscopy were used to determine the silanol content in the most common mesoporous ordered silicas: MCM-41, MCM-48, SBA-15 and SBA-16. In addition, a spray dried MCM-41 and an ethene bridged PMO are investigated. The results are compared with a commercial chromatographic silica (Nucleosil). The complete distribution of surface and bulk silanols, and of isolated, geminal and vicinal silanols for all these materials is presented. A distinction is made between the total silanol number and the reachable or surface silanol content. The latter is determined by controlled reactions with simple silanes. All mesoporous ordered silicas, and especially the thick walled SBA-type materials and the PMO contain a surprisingly high amount of total silanol sites, albeit that up to 90% of these silanols are buried inside the walls and are not reachable for small silanes.